Scissor lift and drive mechanism therefor

ABSTRACT

A scissor lift having upper and lower platforms and actuated by a cable reeved so as to pull at least one set of the ends of the scissor together at its extended position and also reeved about a cam roller interposed between the arms and working toward the pivot connection during the portion of the motion in which the lift begins to extend.

[ Jan. 15, 1974 United States Patent [1 1 Coad et a].

FOREIGN PATENTS OR APPLICATIONS SCISSOR LIFT AND DRIVE MECHANISMTHEREFOR 8/1964 Germany.............................. 187/18 Inventors:George L. Coad, Walnut Creek;

James C. Carter, San Jose, both of Calif.

Primary ExaminerRichard A. Schacher Atl0rney-Paul D. Flehr et al.

ABSTRACT [22] Filed:

211 Appl. No.: 265,704

A scissor lift having upper and lower platforms and actuated by a cablereeved so as to pull at least one set if 7 g g of the ends of thescissor together at its extended posi nt. 254 1/8 tion and also reevedabout a cam roller interposed tween the arms and working toward thepivot connec- Field of Search.......................

tion during the portion of the motion-in which the lift begins toextend.

3 Claims, 7 Drawing Figures SCISSOR LIFT AND DRIVE MECHANISM THEREFORBACKGROUND OF TI-IE INVENTION include various jacking operations, suchas auto jacks,

platform and container lifts and the like. Inherent in the motion cycleof the scissor lift or lazy tong mechanism, is difficulty of applying asingle actuating drive to any single moveable part thereof withouthaving, at one limit of travel of that part, an approach sufficientlyclose to a dead center and consequent low mechanical advantage that therequired actuating force becomes unacceptably high. This causes highstress in the apparatus and requires a drive mechanism capable of powerdelivery sufficiently high to overcome the low mechanical advantage,resulting in higher than desirable power and structure requirements aswell as increased costs. In addition, conventional lift drive mechanismsare so constructed as to add to the load carried by the device and it istherefore necessary to employ heavier structural elements to withstandthe additional load from the drive mechanism itself. There is therefore,a need for a new and improved scissor lift and drive mechanism therefor.

SUMMARY OF THE INVENTION AND OBJECTS In general,.it is the object of'the present invention to provide an improved scissor lift mechanism anddrive therefor which will overcome the above limitations anddisadvantages. I

Another object of the invention is to provide a scissor lift and driveof the above character having a combination of driving elements whichtogether provide for a substantially constant and compensated mechanicaladvantage between the full limits of. travel of the lift so that thevariable and torque requirements of prior art devices are eliminated.

Another object is to provide a scissor lift and drive mechanism thereforof the above character which reduces the structural load stress of thescissor mechanism itself.

Another object of the invention is to provide a scissor lift and drivemechanism of the above character which is noncritical in alignment andwhich remains substantially self-aligning after assembly.

In general, the foregoing objects are achieved by providing a scissorlift mechanism having a base platform and elevatable platforminterconnected 'by sets of spaced pairs of scissor arms pivotedintermediately their ends and pivotally connected to each platform suchthat movement of the respective platforms is accomplished by collapse ofthe scissor arms about their pivots, one side of the ends of the scissorarms being connected for sliding motion along each of the platforms toaccomodate the opening of the scissors while maintaining the platformsin spaced parallel positions. A combination cable and roller cam drivecharacterized by a substantially constant mechanical advantage isemployed for raising and lowering the platforms with respect to eachother. The cable is driven by a winch roller mounted .on one of theplatforms and reeved to pass over roller pulleys connected to each ofthe ends 2 of scissor arms at one platform. The roller cam is positionedto drive the closed sides of the scissor arms apart byworking toward thepivotinterconnecting each of the scissor arms with decreasing mechanicaladvantage BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevationalview partly in cross section of the scissor lift and drive mechanismconstructed in accordance with the present invention and shows the samein a partially raised position.

FIG. 2 is a top plan view partly in cross section and partly broken awaytaken along the lines 2--2 of FIG. 1.

FIG. 3 is a cross-section view taken along the lines 3-3 of FIG. 1.

FIG. 4 is a cross-sectional view taken along the lines 4-4 of FIG. 1.

FIG. 5 is a cross-sectional view taken along the lines 5-5 of FIG. 1.

FIG. 6 is a cross-sectional view taken along the lines 6-6 of FIG. 1. 1

FIG. 7 is an elevation view similar to that of FIG. 1 showing thescissor lift mechanism of the present invention in a raised position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Y which it is desiredto maintain in spaced parallel planes and to separate or lift the upperplatform 12 with respect to the lower. Means are provided for formingfirst and second sets 14, 16 of scissor arms 18, 20, 22, 24interconnected on opposite sides of each of the platforms. As will beevident from reference to FIG. 2 each of the scissor arms and associatedcable and pulley assemblies which will be hereinafter described aresubstantially identical and therefore the description herein will begiven with reference to a single scissor arm 14 and associated drivetherefor, the description being sufficient to clearly pertain to each ofthe sets of scissor arms. FIG. 3 shows a detail view of the pivotconnection between the arms of each set, which includes first and secondbearings 26,28 mounted on each end of a spacing shaft 30 together withwashers 32, 34, 36 interposed between the arms and the shaft to provideadequate clearance. I

One side of each pair of scissor arm pairs is connected to a fixed pivotto the respective platform as indicated 38, and 40. FIG. 4 shows theupper one of the connections as an example of the construction andpreferrably incudes a roller pulley 42 journaled in a bearing 44 andsupporting the respective arm 22 on a second bearing 46 that both theassociated arm and the roller are free to rotate, the roller alsoserving together with spacer block 48 as a structural spacer for thatpair of arm ends.

As shown in FIG. 2 the lower connection is illustrated and includes adouble gear chain drive 50 connected through a worm gear transmission 52to an electric motor 54 to provide sufficient reduction and adequatetorque to permit use of common small electric motor having a rotaryshaft output. The worm gear drive also serves as an automatic brakingfunction since it cannot be backdriven. Together, the motor, wormgearing, chain drive and roller make up a cable winch drive to which thedrive cables of the lift mechanism of the present invention areattached.

Each of the other ends of the scissor arms is slidably connected to therespective platform for reciprocating motion thereon as shown in FIG. 5.Thus, a slot 60 is provided in each side of the upper platform 12 foraccepting a sliding block or bearing 62 into which a roller pulley 64 ismounted on suitable bearings. Other bearings 66 provide a connection toa respective end of the associated scissor arm. The lower scissor arm issimilarly connected so that the upper platform is free to be elevated,the scissor mechanism serving to maintain the platforms parallelinterconnected for motion towards and away from each other, theassociated change in spacing of the respective upper arms and lower armsbeing accomodated by movement by sliding blocks in the respective slots.

Referring now particularly to FIGS. 1 and 6, means is provided forforming a cam roller 70 for wedging one side of the collapsed scissorarms apart during the initial portion of the lift cycle and consists ofa roller pulley 72 having a pair of roller bearings 74, 76 mounted ateach end thereof. Each pair of roller bearings 74, 76 are spaced apartby a space block 78 such that each outer bearing 76 bears upwardly uponthe outer one of the respective arms while the inner bearing 74 bearsdownwardly upon the inner ones of the respective arms. Oversized washers80 are mounted on the outside of the end bearings to maintain lateralpositioning of the cam roller by riding along the outside of the outerarms. Since the cam roller is supported in each of the four bearings 74,76 it is also free for rotation itself and serves thereby as a moveableroller pulley as will be described.

A flexible cable drive is provided and includes a pair of suitablestretch-free wire cables 82, 84 attached to the winch roller and reevedfirst around the cam roller in a sense such as to cause the cam rollerto work towards or away from the pivot connection 30 of the scissorarms. From there each cable is reeved back around the roller pulley 42above the winch which serves as a turning block. The cable is thenreeved around moveable roller pulley 64 and then set to a dead eye suchas a member 86 mounted to the upper platform and through which the cableend terminates in a threaded swaged connection 88 adapted for adjustabletensioning by a pair of lock nuts 90.

Limit switches 92 and 94 are provided at each end of the desired travelof pulley 64 in the slots and are adapted to be contacted by the rolleras the same reaches the limit of travel in either direction to open thecircuit to motor 54 and cause the mechanism to stop.

The operation of the lift and drive of the present invention is bestseen from reference to FIGS. 1 and 7. As shown in FIG. 1 in the firstportion of the cycle the cable works primarily to drive cam rollertoward the scissor pivot of each of the rollers, wedging the scissorarms upwardly. Since the amount of upward motion is small compared tomovement of the cam roller, the mechanical advantage is large. As theroller nears the pivot, however, its rate of movement and mechanicaladvantage is reduced. During its motion, the cam roller also serves as aone of two moving blocks in a block and tackle formed by itself andmoveable roller pulley 64, as well as a turning block, the later beingits primary function as it approaches the scissor pivot. As bestillustrated in FIG. 7 in the later part of the cycle the upper moveableroller operates as a single part block and tackle with additionalmechanical advantage due to reduction of the increment of lift movementas the same nears its upper limit of travel.

In summary, the cam roller urges the platform upward with largemechanical advantage due to the wedge action in the first part of thecycle and a gradual transition occurs when the cam roller and moveablepulley operate as a double block and tackle together with wedges orinclined plane action both of the cam roller and of the sliding block towhich the moveable pulley is mounted. Finally, the action of the camroller diminishes and the mechanical advantage residing in the singlemoveable block and. the sliding wedge or inclining action of the movablepulley.

To those skilled in the art to which the present invention pertains itwill be apparent that many modifications and changes of the presentstructure will fall within the general scope of the present invention byway of example, both the movable platform and winch drive may be locatedon the same platform and aside from being connected to the element withwhich they are designed to move none of the pulleys need be coincidentwith the shafts on which they are shown as located in the foregoingdescription.

In the present embodiment the location of the pulleys was deemedexpedient as a matter of design rather than necessity. In addition itwill be apparent that the roller cam may be arranged for operation suchthat it travels together with the one part block and tackle or pulleysystem or it may be arranged to operate in opposition thereto withoutdeparting from the spirit and scope of the present invention. Thus suchreversals of parts and changes in layout are to be taken as within thescope of the teachings of the present invention and claims.

We claim:

1. A scissor lift and drive mechanism comprising means forming first andsecond platforms, means forming first and second pivotallyinterconnected pairs of scissor arms having first ends pivotally mountedin spaced parallel planes on said first platform, said last named meansincluding means interconnecting at least one first end of each of saidscissor arms for sliding movement toward the other first end to therebyclose the gap therebetween and to extend the second ends of said armsaway from the first ends and from said first platform to provide therebya lifting action, said second platform being mounted on the second endsof said arms to be lifted thereby and including means mounting at leastone of said second ends of said arms for sliding motion on saidplatforms to and from the other second end to accomodate closing of thesecond ends thereof as the same are activated into extended position, acam roller interposed between the first and second ends of each of saidscissor arms on one side thereof and having means thereon for permittingsliding motion along said arms toward the extremeties thereof when thesame are lowered and toward the pivotal connection therebetween as thesame are raised, first pulley means disposed on said first platformapproximate to said other first end, second pulley means disposed on thesecond platform at a position approximate said other second end, saidroller cam being disposed on that side'of said scissor lift mechanismaway from said first and second pulley means, third pulley meansconnected to said one second end, means for driving one of said first orthird pulleys, cable means connected to said driven pulley and reevedabout said cam roller and said second pulley'means and thence about saidthird or first pulley means, respectively, and means connecting the endof said cable to the first or second platform, respectively.

2. A scissor lift and drive mechanism comprising means forming first andsecond platforms, means forming first and second pivotallyinterconnected pairs of scissor arms having first ends pivotally mountedin spaced parallel planes on said first platform, said last named meansincluding means interconnecting at least one first end of each of saidscissor arms for sliding movement toward the other first end to therebyclose the gap therebetween and to extend the second ends of said armsaway from the first ends and from said first platform to provide therebya lifting action, said second platform being mounted on the second endsof said arms to be lifted thereby and including means mounting at leastone of said second ends of said arms for sliding motion on said platformto and from the other second end to accomodate closing of the secondends thereof as the same are activated into extended position, a camroller interposed between the first and second ends of each of saidscissor arms on one side thereof and having means thereon for permittingsliding motion along said arms toward the extremeties thereof when thesame are lowered and toward the pivotal connection therebetween as thesame are raised, movable pulley means attached to at least one of thesecond ends of said scissors arms, cable means having a fixed endattached to the first or second platform and a running end reeved aboutsaid cam roller and said movable pulley means so as to cause each of thesame to work inwardly toward the pivotal connection of said arms as thesame is actuated, and means on the second or first platform,respectively, to apply a tension force along the length of said cablemeans for actuating said arms.

3. A scissor lift as in claim 2 wherein said cam roller is arranged tooperate in the same direction as said moveable pulley.

1. A scissor lift and drive mechanism comprising means forming first andsecond platforms, means forming first and second pivotallyinterconnected pairs of scissor arms having first ends pivotally mountedin spaced parallel planes on said first platform, said last named meansincluding means interconnecting at least one first end of each of saidscissor arms for sliding movement toward the other first end to therebyclose the gap therebetween and to extend the second ends of said armsaway from the first ends and from said first platform to provide therebya lifting action, said second platform being mounted on the second endsof said arms to be lifted thereby and including means mounting at leastone of said second ends of said arms for sliding motion on saidplatforms to and from the other second end to accomodate closing of thesecond ends thereof as the same are activated into extended position, acam roller interposed between the first and second ends of each of saidscissor arms on one side thereof and having means thereon for permittingsliding motion along said arms toward the extremeties thereof when thesame are lowered and toward the pivotal connection therebetween as thesame are raised, first pulley means disposed on said first platformapproximate to said other first end, second pulley means disposed on thesecond platform at a position approximate said other second end, saidroller cam being disposed on that side of said scissor lift mechanismaway from said first and second pulley means, third pulley meansconnected to said one second end, means for driving one of said first orthird pulleys, cable means connected to said driven pulley and reevedabout said cam roller and said second pulley means and thence about saidthird or first pulley means, respectively, and means connecting the endof said cable to the first or second platform, respectively.
 2. Ascissor lift and drive mechanism comprising means forming first andsecond platforms, means forming first and second pivotallyinterconnected pairs of scissor arms having first ends pivotally mountedin spaced parallel planes on said first platform, said last named meansincluding means interconnecting at least one first end of each of saidscissor arms for sliding movement toward the other first end to therebyclose the gap therebetween and to extend the second ends of said armsaway from the first ends and from said first platform to provide therebya lifting action, said second platform being mounted on the second endsof said arms to be lifted thereby and including means mounting at leastone of said second ends of said arms for sliding motion on said platformto and from the other second end to accomodate closing of the secondends thereof as the same are activated into extended position, a camroller interposed between the first and second ends of each of saidscIssor arms on one side thereof and having means thereon for permittingsliding motion along said arms toward the extremeties thereof when thesame are lowered and toward the pivotal connection therebetween as thesame are raised, movable pulley means attached to at least one of thesecond ends of said scissors arms, cable means having a fixed endattached to the first or second platform and a running end reeved aboutsaid cam roller and said movable pulley means so as to cause each of thesame to work inwardly toward the pivotal connection of said arms as thesame is actuated, and means on the second or first platform,respectively, to apply a tension force along the length of said cablemeans for actuating said arms.
 3. A scissor lift as in claim 2 whereinsaid cam roller is arranged to operate in the same direction as saidmoveable pulley.